1. Field of the Invention
The present invention relates to an internal combustion engine and a method of controlling the internal combustion engine.
2. Description of the Related Art
Japanese Patent Publication No. 2000-230445 (JP-A-2000-230445) describes an internal combustion engine having a plurality of cylinders divided into two cylinder groups, and exhaust pipes, connected in association with each cylinder group, which are joined downstream into a common exhaust pipe. In the described internal combustion engine, a three-way catalyst is disposed in the exhaust pipes connected to each cylinder group, and another three-way catalyst is disposed in the common exhaust pipe. A control is performed to correct the amount of fuel injected from a fuel injection valve (hereinafter “fuel injection amount”) so that the air-fuel ratio is maintained at the target air-fuel ratio, based on the air-fuel ratio detected by air-fuel ratio sensors (indicated as 13L and 13R in FIG. 1 of the cited reference, hereinafter “upstream sensors”) disposed upstream of the upstream three-way catalysts. According to the cited document, when a prescribed condition is satisfied, fuel vapor is discharged to the intake pipe from a canister that holds evaporated fuel generated in the fuel tank.
Because the fuel vapor that is discharged to the intake pipe from the canister is ultimately taken into a cylinder and combusted, the fuel vapor affects the air-fuel ratio. In the internal combustion engine described in JP-A-2000-230445, a correction coefficient that corrects the fuel injection amount to maintain the air-fuel ratio at the target air-fuel ratio is determined based on the air-fuel ratio detected by the upstream air-fuel ratio sensors. The proportion of fuel vapor included in the gas ejected from the canister into the intake pipe (hereinafter “fuel vapor concentration”) is determined based on the correction coefficient, and the fuel injection amount is controlled to maintain the air-fuel ratio at the target air-fuel ratio, based on the determined fuel vapor concentration.
However, in the internal combustion engine described in JP-A-2000-230445, in order to increase the temperature of the downstream three-way catalyst, there is a need not only to supply a relatively large amount of fuel and air to the three-way catalyst, but also to make the air-fuel ratio of the exhaust gas flowing into the three-way catalyst be the stoichiometric air-fuel ratio. A known means for satisfying this need is to cause combustion in one cylinder group at an air-fuel ratio that is richer than the stoichiometric air-fuel ratio and cause combustion in the other cylinder group at an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, so that the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is the stoichiometric air-fuel ratio.
When causing combustion in one cylinder group at an air-fuel ratio richer than the stoichiometric air-fuel ratio and causing combustion in another cylinder group at an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter “rich-lean operation”), the air-fuel ratio of exhaust gas flowing into the upstream three-way catalyst may be rich or lean. Therefore, even if an attempt is made to maintain the air-fuel ratio in each of the cylinder groups at the stoichiometric air-fuel ratio based on the air-fuel ratio detected by the upstream sensors, it is not possible to maintain the air-fuel ratio accurately at the stoichiometric air-fuel ratio. As a result, it is known that the air-fuel ratio for each of the cylinder groups is maintained at the stoichiometric air-fuel ratio based on the air-fuel ratio detected by an air-fuel ratio sensor disposed in the upstream from three-way catalyst that is downstream from the point of joining of the exhaust gas from one cylinder group and the exhaust gas from the other cylinder group (referred to as the downstream sensor and assigned the reference numeral 16 in JP-A-2000-230445).
In the internal combustion engine described in JP-A-2000-230445, the fuel vapor concentration is determined based on a correction coefficient that corrects the fuel injection amount, so that the air-fuel ratio is maintained at the stoichiometric air-fuel ratio. When rich-lean operation is not performed (hereinafter “normal operation”), the fuel vapor concentration is determined based on a correction coefficient with respect to the fuel injection amount determined based on the air-fuel ratio detected by the upstream sensor, and during the rich-lean operation, the fuel vapor concentration is determined based on a correction coefficient with respect to the fuel injection amount determined based on the air-fuel ratio detected by the downstream sensor.
The fuel vapor concentration detection during normal operation of the internal combustion engine is performed at fixed time intervals. When this is done, the determined fuel vapor concentration is generally stored as a learned value, and the learned value of fuel vapor concentration that was stored the immediately preceding cycle is used to determine the fuel vapor concentration in subsequent cycles. In this case, immediately after the operation of the internal combustion engine switches from normal operation to rich-lean operation, the fuel vapor concentration is determined using the learned value of fuel vapor concentration determined when performing normal operation. However, because the fuel vapor concentration is determined during normal operation using the upstream sensor output, when the operation of the internal combustion engine switches to rich-lean operation, the fuel vapor concentration is determined based on the learned value of fuel vapor concentration determined based on the output of the upstream sensor and on the output of the downstream sensor.
Given the above, even if the upstream sensors and downstream sensor are of the same type, and especially if they are of different types, there is an inherent difference in the output characteristics thereof. Therefore, when the operation of the internal combustion engine switches from normal operation to rich-lean operation, it is not possible to accurately determine the fuel vapor concentration by determining the fuel vapor concentration during rich-lean operation using the learned value of fuel vapor concentration determined during normal operation.